Better delineation of the viral targets and pathways involved in sexual transmission of HIV-1 is crucial to the global effort to develop a protective vaccine, and to other preventive strategies. Studies to date suggest that viruses found in newly infected individuals are commonly macrophage-tropic and non-syncytium inducing, irrespective of the viral phenotypes found in their corresponding sexual partners. Moreover, genetic studies in this setting have found evidence of a stronger pressure to conserve sequences in gp12O than in gp41, p17, or nef, suggesting that a selective mechanism may be involved in sexual transmission. This possibility is further supported by our recent finding that minor variants of the HIV-1 populations present in seminal fluids of chronically infected men are preferentially transmitted to their male or female sexual partners. These preliminary but intriguing results emphasize the need to define the initial cellular targets for HIV-1 in the genital mucosa and the subsequent pathway of viral spread. However, these unresolved questions are extremely difficult to pursue experimentally in humans. We therefore propose to examine several of the issues concerning sexual transmission in a rhesus macaque model using both SIV and SHIV chimeric viruses. Specifically, we will attempt to define the identity of the first cell(s) infected by SIV in the vaginal and cervical mucosa of macaques using in situ PCR coupled with immunohistochemistry. Employing culture and PCR methods, we will also track the kinetics and the pathway of SIV spread from the genital mucosa to the draining (internal iliac) lymph nodes before systemic dissemination. In addition, we will perform a series of experiments to examine directly the issue of selective transmission. Mixtures of SIV variants with known differences in their biological phenotypes will be inoculated both mucosally and intravenously into macaques to determine whether virus with a particular biological property (e.g., macrophage-tropism) will be preferentially selected in animals inoculated mucosally. Similar experiments will also be performed using mixtures of SHIV chimeric viruses with divergent biological phenotypes. Lastly, we will determine the impact of early mucosal immune responses, particularly cytotoxic T lymphocytes, in preventing the establishment of mucosal infection or in aborting the subsequent spread of the virus. We believe that the proposed studies will contribute considerably to our understanding of sexual transmission of HIV-1.